The images you're referring to are enhanced (stretched) color, not false color. According to the official release, the hollows are "very bright and have a blue color relative to other areas of Mercury." As one image caption from NASA explains, "the enhanced-color locator image emphasizes the high reflectance and relatively blue color."

The images are enhanced (stretched) color, not false color. According to the official release, the hollows are "very bright and have a blue color relative to other areas of Mercury." As one image caption from NASA explains, "the enhanced-color locator image emphasizes the high reflectance and relatively blue color."